1. Field of the Invention
The present invention relates to an organic electrolytic solution and a lithium battery using the same, and more particularly, to an organic electrolytic solution to prevent batteries from swelling and to improve the reliability thereof, and a lithium battery using the organic electrolytic solution.
2. Description of the Related Art
As portable electronic devices, such as camcorders, mobile phones, notebook computers, and the like, become smaller and more lightweight while at the same time becoming functionally more powerful, considerable research is being conducted into batteries as a driving source for these portable devices. In particular, rechargeable lithium secondary batteries have three times greater energy densities per unit weight and higher charging rates relative to conventional lead batteries, nickel-cadmium (Ni—Cd) batteries, nickel-hydrogen batteries, nickel-zinc batteries, and the like. For these reasons, lithium secondary batteries have attracted greater attention, and intensive research thereinto has been conducted.
In lithium ion batteries, a lithium complex oxide such as LiNiO2, LiCoO2, LiMn2O4, LiFePO4, LiNixCo1-xO2, Ni1-x-yCoxMnyO2 or the like, is used as cathode active materials, and lithium metal, lithium metal alloys, carbonaceous materials, graphitic materials, etc., are used as anode active materials. There are two types of electrolyte: liquid electrolyte and solid electrolyte. The use of liquid electrolyte raises safety concerns, such as a risk of fire and explosion of batteries due to evaporation of the liquid electrolyte. To address these problems, using solid electrolytes instead of liquid electrolytes has been suggested. Solid electrolytes are unlikely to suffer from leakage and are easy to process. Due to these advantages of the solid electrolyte, intensive research into such a solid electrolyte, particularly, a polymer solid electrolyte, is currently being undertaken. Existing polymer solid electrolytes may be classified into a full solid electrolyte and a gel electrolyte containing an organic electrolytic solution.
General lithium batteries operate at a high working voltage and are incompatible with conventional aqueous electrolytic solution. The most probable reason for this is that their lithium anode reacts vigorously with the aqueous solvent of the electrolyte. Accordingly, organic electrolytic solutions containing a lithium salt in an organic solvent are used in lithium batteries. An efficient organic solvent for organic electrolytic solutions has greater ionic conductivity, larger dielectric constant, and lower viscosity. However, these requirements cannot be fully satisfied with a single organic solvent, so that a mixture of a high dielectric constant organic solvent and a low dielectric constant organic solvent, a mixture of a high dielectric constant organic solvent and a low viscosity organic solvent, and the like, have been used.
U.S. Pat. No. 6,114,070 and No. 6,048,637 disclose the use of a solvent mixture of a linear carbonate and a cyclic carbonate, for example, a mixture of dimethyl carbonate or diethyl carbonate and ethylene carbonate or propylene carbonate, to improve the ionic conductivity of the organic solvent. However, such solvent mixtures may be used only at 120° C. or less. If the temperature is higher, the solvent mixture vaporizes and the battery swells due to the gas generated from the solvent mixture.
U.S. Pat. No. 5,352,548, No. 5,712,059, and No. 5,714,281 disclose the use of 20% or greater of vinylene carbonate as a main solvent. However, vinylene carbonate has a lower dielectric constant than ethylene carbonate, propylene carbonate, and γ-butyrolactone and leads to considerably poor charge/discharge and high-rate characteristics when used as a main solvent in batteries.
U.S. Pat. No. 5,626,981 discloses the formation of a surface electrolyte interface (SEI) on the surface of a cathode during initial charging/discharging by using a vinylene carbonate-added electrolytic solution. Japanese Laid-open Patent Application No. 2001-217001 discloses an effective non-aqueous electrolytic solution containing a phosphagen derivative that is nonflammable and provides good low temperature characteristics and interfacial resistance characteristics. Japanese Laid-open Patent Application No. 2001-176548 discloses the use of sulfuric ester as an additive to minimize the first cycle irreversible capacity of a lithium ion battery and to maintain a low temperature cycling capacity.
However, when the batteries in the above-described patents remain at a high temperature, the batteries swell beyond an allowable thickness limit such that their cases deform and cannot be used any longer.
Japanese Laid-open Patent Publication No. hei 10-223257 discloses the use of a nonflammable electrolytic solvent containing phosphite ester having the formula of (R1O)P(═O)(OR2)(R3) where R1, R2, and R3 are C1–C4 alkyl, to improve electrical conductivity and reduce reductive decomposition for the improved charge/discharge efficiency, and lifespan of batteries.
U.S. Pat. No. 6,200,701 discloses the use of phosphite ester having the formula of (R1O)P(═O)(OR2)(R3), where R3 is a hydrogen atom, and at least one of R1 and R2 is a hydrogen atom or a saturated C1–C13 organic compound, to improve the charging/discharging efficiency, lifespan, and first cycle irreversible capacity of batteries.